Remote electronic identification devices typically consist of remotely disposed transponders and an interrogator unit. The operating range of such devices depends on the underlying architecture of the transponder unit. In U.S. Pat. No. 4,333,072 to Beigel, for example, a remote identification system consists of a probe circuit (interrogator) operating in close proximity to an implant circuit (transponder). The implant circuit is powered when the probe circuit is brought close to the implant, producing a voltage by induction across a coil in the implant. Information from the implant is transferred to the probe by varying the inductive load on the implant's coil and detecting such variations in the probe. This mode of operation requires that the probe be very closely spaced from the implant, thus greatly limiting the range of operation of such devices.
Other remote identification systems use radio frequency signaling to provide the communication link between interrogator and transponder. In one class of these radio frequency identification (RFID) devices, the RF signal includes a power signal that is transmitted to the transponder device. This power signal charges up a power supply capacitor in the transponder, which serves as the power source for the transponder. The transmission of data from the transponder involves the generation and transmission of radio frequency energy. The power supply capacitor must be of sufficient size to provide adequate power for such transmissions. Such transponders allow reading distances on the order of 2 meters. This type of transponder, however, is not practical in applications requiring small size; e.g. livestock identification by subcutaneous implantation of the device.
In a third category of RFIDs, an alternative approach combines features from the above two designs. An interrogator transmits an RF signal which includes a power signal. The voltage induced across the transponder coil by the received power signal is sufficient to operate the transponder circuitry, but not sufficient to generate its own radio signal. A capacitor coupled in parallel with the coil forms a tank circuit, whose Q is varied by varying a resistive or a capacitive load placed across the tank circuit. This changes the tuning of the tank circuit, resulting in variations in the reflected signal which can be detected by the interrogator. Thus, the transponder is able to convey its data to the interrogator simply by modulating the resistive load accordingly and allowing the interrogator to detect variations in the reflected signal.
This scheme has advantages over the above two designs. First, the reflected signal is detectable over a distance of roughly 1 meter. Thus, close proximity operation of the interrogator to the transponder is not required as in the case of the Beigel device. Second, since the transmitted power signal is not being stored in the transponder, there is no need for a bulky power supply capacitor thus permitting a smaller form factor package.
Typically, RFIDs contain writable nonvolatile memories and include a power on reset circuit that prevents operation of the device until the voltage level of the device reaches a certain level. In order to guarantee reliable data transmissions, the practice has been to set the level of the reset circuit to the highest level for operating the nonvolatile memory. This artificially limits the read range, since the reset voltage is much higher than is truly needed to operate the memory.
An improvement would be to increase the operating range of this third category of RFID devices, to provide performance comparable to RFID devices which utilize supply capacitors. What is needed therefore is an RFID device which offers the small package outline that is possible by eliminating a supply capacitor and which features long range read capability.